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Author: search.filters.author.Gaewdang, ThitinaiSubject: search.filters.subject.CdSxTe1-x thin films

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    Structural, optical and photoconductive properties of thermally evaporated CdSxTe1-x thin films
    (2017-10-01)
    Gaewdang, Thitinai
    ;
    Wongcharoen, Ngamnit
    The formation of a CdS<inf>x</inf>Te<inf>1-x</inf> layer by the interdiffusion of S into the CdTe and of Te into CdS films occurs during the fabrication of CdS/CdTe thin film solar cells. The CdS<inf>x</inf>Te<inf>1-x</inf> layer is thought to be important because the real electrical junction is located in this interdiffused CdS<inf>x</inf>Te<inf>1-x</inf> region. Thus, it is important to have a full understanding of the physical properties of CdS<inf>x</inf>Te<inf>1-x</inf> alloy thin films. In this study, CdS<inf>x</inf>Te<inf>1-x</inf> thin films with composition 0 ≤ x ≤ 1 were prepared by thermal evaporation method on glass substrate using powder of pure CdS and CdTe compounds pressed in pellet form as the source in a vacuum of 5.5 10<sup>-5</sup> mbar. X-ray diffraction (XRD) revealed that the films exhibited a cubic zincblende structure with the preferred orientation of (111) plane when x < 0.2. However, when x ≥ 0.8, they had a hexagonal wurtzite structure with the preferred orientation of (002) plane. For the composition 0.2 ≤ x ≤ 0.6, the cubic and hexagonal phases coexisted in the system and the films became less preferentially oriented. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the morphological features of the samples. The transmission spectra of the films were studied using a double beam spectrophotometer in the wavelength range of 300-900 nm. Optical band gap value of the films was determined from the transmittance spectra. The variation of band gap (E<inf>g</inf>) with composition (x) of the films was in good agreement with the quadratic form, giving a bowing parameter of (b) =1.85 eV. From the transient photoconductivity measurements, persistent photoconductivity (PPC) behavior was observed. The decay current data fit better with multiple exponential functions, resulting in five slow decay times. The longest carrier lifetime of approximately 6,000 s was observed in the films with x = 0.8. Density of trap states corresponding to decay time was also evaluated from the decay current data.
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    Physical properties of CdSxTe1-x thin films prepared by close spaced sublimation method
    (2017-01-01)
    Gaewdang, Thitinai
    ;
    Wongcharoen, Ngamnit
    CdS<inf>x</inf>Te<inf>1-x</inf>(0 ≤ x ≤ 1) thin films were prepared on clear quartz glass substrate by close spaced sublimation method using stoichiometric mixed powders of pure CdS and CdTe compounds. Crystal structure of CdS<inf>x</inf>Te<inf>1-x</inf>thin films was cubic structure with a preferential orientation of (111) plane when the S mole ratio less than 0.2. However, For the composition 0.2 ≤ x ≤ 0.8, the cubic and the hexagonal phases were found to coexist in the system and the films became less preferentially oriented whereas CdS thin films formed in the hexagonal phase with a preferential orientation of (002) plane. SEM micrographs showed the grain size decreased when the S content increased. As the S content increased, the energy gap value of CdS<inf>x</inf>Te<inf>1-x</inf>thin films varied from 1.48 eV (for CdTe) to 2.25 eV (for CdS). Dark electrical sheet resistance of CdS<inf>x</inf>Te<inf>1-x</inf>thin films decreased as a function of S content to minimum value about 2.24×10<sup>7</sup>Ω/sq for x=0.8 and then slightly increased to 4.13×10<sup>7</sup>Ω/sq for x=1.0. From the transient photoconductivity measurements, the highest photosensitivity value about 60.0 was obtained for the films with x = 0.8.